This invention relates to the preparation of glass containers for thermoplastic membrane sealing. Heat activated membrane seals for plastic containers are common in the packaging and canning art. Generally a membrane, which may be a laminate of aluminum foil and a thermoplastic polymer, is pressed on the rim of a plastic container and heated to form polymer-polymer adhesive contact. The problem with this method when used with soda-lime-silica glass containers is that the failure of the seal will generally occur within a few days of sealing due to poor initial adhesion between the glass and the polymer or poor durability of the adhesion.
Substantially improved adhesion between a membrane seal and a container finish is obtained by subjecting the finish to a high temperature treatment which modifies the finish surface. This high temperature treatment preferably is conducted immediately after the containers are formed and comprises contacting the finish with a decomposible fluorine-containing compound, or a compound which decomposes to form a metal oxide, or a compound which decomposes to form a sulfur oxide, sulfur oxides and combinations of the above. These treating materials may be used in any order or simultaneously.
A sealing closure for the container is formed by pressing a membrane comprising a thermoplastic film onto the container finish and heating to form a glass/plastic adhesive bond. The membrane desirably may be an aluminum foil-thermoplastic film laminate. It may also be a polymer sheet, a laminate of polymer, a paper/polymer laminate or a laminate of one or more layers of polymers, metal foil, and paper.
This described technique provides a reliable and secure closure so long as the container finish remains uncontaminated. However, during cold end treatment of the containers after annealing, it was found that the treating material, typically a polymer, stearate or silicone, would occasionally be deposited on the container finish to the extent that erratic sealing results were obtained.